A study led by the University of Barcelona has identified the process in the brain which strengthens or weakens memories of our daily events while we sleep. According to the study published in the journal The Journal of Neuroscience, the brain reactivates networks of interrelated memories during sleep, instead of doing so with isolated memories. Also, researchers show that in this memory reactivation process, the brain also promotes the forgetting of less important and less established information in the network of memories.

Participants in the study are Javiera Oyarzún and ICREA lecturers Ruth de Diego Balaguer and Lluís Fuentemilla, researchers of the Group on Cognition and Brain Plasticity of the UB, from the Bellvitge Biomedical Research Institute (IDIBELL) and the Institute of Neurosciences of the UB. The study also has the collaboration of Joaquín Morís, from the University of Oviedo, and David Luque, from the University of New South Wales (Australia).

One of the most relevant questions in the field of memory is to know how to select experiences that are going to be remembered in the future. Scientific research studies have proved that during the sleeping process, the brain reactivates some of the encoded memories so that they are stored. In order to delve into this memory purging mechanism, researchers used an experimental technique to induce the memory activation artificially.

This technique relates a sensory stimulation to a memory in order to induce its reactivation artificially during sleep. In the study, participants had to memorize fifteen locations –in a grid- of pairs of 147 identical cards with pictures of animals, means of transport or musical instruments. In the first set, cards were in different positions inside the grid and the second set had one of the cards in a different place regarding the first one, while the other was in the same place. Therefore, the task made participants form two different sets of memory traces which shared a common element: the position of one of the cards in each pair. In addition, in the second set, card pairs were attached sounds related to the picture they had so that researchers could induce the memory activation for these positions showing sounds while participants were sleeping in the laboratory.

Therefore, while participants were sleeping, researchers showed half of the sound stimulus randomly selected (animal sounds, transport, or musical instruments) and reactivated memories of the positions of the cards that were related to each sound. However, when participants woke up, researchers evaluated the memory of the place of the first set of cards that had not been related to any sound but shared an element (they were related to each other) with those reactivated and those not reactivated during the sleeping process.

To manipulate the relation between the interrelated memories of the first set with those of the second set, researchers used two groups of participants. One group learnt both sets of cards consecutively (continuous group) so that both sets of cards were strongly related to each other. A second group learnt the second set of cards three hours after having learnt the first set (delay group). Therefore, both sets of memories were poorly related.

Results revealed that the continuous group remembered better the memories of the first set of cards that were related to those that had been reactivated during sleep. That is, those reactivated memories through sounds, in the second set, helped them remembering the location of the related pairs in the first set (same picture and position of the first card). “In the first group, memories were strongly interrelated, since they were encoded in a very close and similar context. This feature allows these memories to benefit from the reactivation of other elements in the network, since they represent solid and important information”, said the supervisor of the study, Lluís Fuen